Regenerative Thermal Oxidizers (RTOs) are commonly used as part of industrial processes to treat polluted air. Specifically, RTOs are commonly used to decompose toxic gases and volatile organic compounds (VOCs) that are discharged in industrial process exhausts.
The basic operation of a typical RTO consists of passing a hot gas stream over a heat sink material in one direction and recovering that heat by passing a cold gas stream through that same heat sink material in an alternate cycle to heat the cold gas stream. The heat sinks comprising such systems often comprise one or more beds of ceramic material configured to absorb heat from the exhaust gas, wherein the captured heat is then used to preheat an incoming process gas stream. Preheating this incoming process gas is important because it raises the temperature of the incoming gas closer to the temperature required for combustion, necessitating less energy to attain combustion. In this way RTOs help to more efficiently destroy air pollutants emitted from process exhaust streams by recovering and reusing heat created by these types of combustion systems.
Due to the high thermal energy recovery rate of many RTOs, they are suited to applications with low VOC concentrations but high polluted air flow volumes. As a result, RTOs are commonly used to control air emissions and pollutants from various industrial processes such as those involving automotive painting, industrial packaging, wood engineering, agricultural drying and waste treatment just to name a few.
Today, most existing RTOs rely on some form of ceramic heat sink to provide regenerative heat transfer, and many forms of such elemental ceramic media are currently available. Elemental ceramic media are often provided in the form of small pieces. Such ceramic media can often be in the form of blocks, commonly referred to as “saddles,” that are combined to form heat exchange media and comprise multiple tubes or similar openings extending through each block, wherein the tubes or openings are configured to allow air to flow through the block. Due to the fact that these ceramic heat sinks are generally large and bulky, they are commonly assembled into one or more towers, where they remain stationary. A series of valves then directs airflow into and out of each tower, or chambers comprising the towers. RTOs comprising this type of arrangement are referred to as “fixed-bed” design RTOs.
Fixed-bed RTOs are known to have some significant disadvantages. First, it is nearly impossible to distribute airflow uniformly throughout each regenerative bed. As a result, “dead spaces” will exist within almost any fixed-bed RTO system. In such dead spaces, the pollutant containing air will not be effectively treated. A stratification effect occurs when entering airflow is not effectively distributed across the entire heat recovery bed. For instance, airflow is not properly distributed in the corners of fixed bed RTOs. A strategy for minimizing the effect of dead spaces has been to significantly enlarge each unit of the RTO system. This enlargement requires the use of larger ceramic heat recovery chambers to create the fixed-beds, which results in a lower heat transferring efficiency. Ceramic saddles are commonly used as a heat transfer media in RTOs and have a shape that is a composite of a ring shape and a saddle shape. Generally speaking, the smaller the heat transfer particle, the more efficient the heat transfer process will be. For example, less one-inch ceramic saddles are needed than two-inch ceramic saddles to achieve the same degree of thermal efficiency.
The second disadvantage of fixed-bed RTOs is that they require a complex valve system to direct air through the RTO chambers resulting in higher construction and operation costs. These valve systems typically move air first in one direction, then in the opposite direction, known as flow reversal, so that the heat from combustion can be captured by the heat sinks, and can then be used to preheat the next batch of pollutant containing air. Perhaps most importantly, the switching mechanisms comprising these valve systems often allow some of the pollutant containing air, which does not reach the heat sinks, to be released untreated. Such releases can account for the majority of pollutants that are allowed to be emitted from RTO treating systems.
What is needed is an RTO system that can distribute airflow uniformly throughout the RTO's heat sink materials thus reducing or eliminating “dead spaces” while also eliminating flow reversal and the need for complex and inefficient valve systems and large ceramic saddles or other large heat sinks.